For the Vermont independent, the choice was a principled one: He said he wanted to fight the election on issues he thought voters cared about, and progressive leaders maintain that Mr. Sanders would’ve done little to help his candidacy by zeroing in on the email issue.

But the decision undoubtedly allowed Mrs. Clinton to hurdle some of the deepest questions about her candidacy and avoid tackling the root causes of why so many voters find her untrustworthy.

“It was a mistake for Sanders not to go after Clinton on the email scandal. That has attracted lots of media attention and Sanders lost an opportunity to question Clinton’s integrity on a variety of topics. He could have used this subject to tie together controversies on the Clinton Foundation, Benghazi, and lecture fees,” said Darrell West, director of governance studies at the Brookings Institution.

While Mrs. Clinton is poised for major victories on Super Tuesday and seems to be on the fast track to the Democratic nomination, a collection of issues has led voters to conclude she’s untrustworthy. In addition to the revelations around her use of a private email account while secretary of state — and the fact that classified information passed through that account, something Mrs. Clinton previously had denied — the Democratic front-runner also has faced questions about whether foreign donors essentially bought political favors through contributions to the Clinton Foundation. There also are questions still swirling about whether Mrs. Clinton has been honest about her actions in the lead-up to and aftermath of the 2012 Benghazi terrorist attacks.

Those and other issues seem to have led many voters to conclude she simply isn’t an honest candidate.

A CNN/ORC International poll released Monday found just 36 percent of Democrats see Mrs. Clinton as the most trustworthy and honest candidate in the Democratic field. Nearly 60 percent said Mr. Sanders is more honest, and other polls have shown similar results.

Mrs. Clinton has struggled to shed that reputation of dishonesty. At a town hall meeting last month, she merely said she has “always tried to” tell the truth and doesn’t think she’s ever lied to the American people.

Despite all of that, Mr. Sanders has focused almost entirely on the issue of income inequality and hasn’t made the Democratic primary election about Mrs. Clinton’s character and honesty.

While he has questioned Mrs. Clinton’s paid speeches to Goldman Sachs and her ties to the so-called Wall Street and political “establishment,” he said in a debate last October that he didn’t believe the email issue was worth discussing.

“The American people are sick and tired of hearing about your damn emails,” Mr. Sanders said at the debate. “Enough of the emails. Let’s talk about the real issues facing America.”

Last month, the senator did say he considered the email scandal to be a “serious issue,” but he failed to bring it up again in subsequent debates and speeches.

“It’s the worst kind of Republican gotcha politics, and in a Democratic primary, playing that up is not going to help you,” said Neil Sroka, communications director for the liberal PAC Democracy for America.

Other political analysts say that whatever short-term benefits Mr. Sanders may have gotten from focusing on Mrs. Clinton’s emails may have been outweighed by the damage done to his own reputation.

“It would be seen as a selfish power grab,” said Michael Cornfield, a political science professor at George Washington University. It “wouldn’t go over well with the many idealists, especially the young, in his base.”